This invention relates to thermally reworkable epoxy resin compositions, and more particularly to thermally reworkable carbamate or carbonate epoxide resin compositions which degrade at temperatures significantly lower than traditional cycloaliphatic epoxy resins.
In the field of electronic packaging and, in particular, the field of integrated circuit (IC) chip interconnection, the desirability of incorporating high input/output (I/O) capability and short IC interconnects, among others, typically has led to the adoption of the flip-chip technique of IC chip interconnection. Generally, the flip-chip technique involves electrically interconnecting an IC chip and a substrate with the use of solder joints which are disposed between the IC chip and the substrate.
As initially practiced, the flip-chip technique oftentimes utilized relatively high cost materials, such as high lead solder and ceramic substrate. However, the desire to reduce costs has prompted the use of less expensive materials, such as flip-chip on board (FCOB), which typically utilizes eutectic solder and organic printed wiring board (PWB). While reducing material costs, the use of FCOB packaged systems has accentuated the problem of coefficient of thermal expansion (CTE) mismatch between the IC chip and the organic substrate of the FCOB, particularly when large IC chips and fine pitch, low profile solder joints are utilized. Due to the CTE mismatch between silicon IC chips (2.5 ppm/xc2x0 C.) and organic substrates, i.e., FR-4 PWB (18-24 ppm/xc2x0 C.), temperature cycle excursions experienced by the FCOB generate tremendous thermo-mechanical stress at the solder joints and, subsequently, can result in performance degradation of the packaged system.
It is also known in the prior art to fill the spaces or gaps remaining between an IC chip and substrate which are not occupied by solder with an underfill composition. The undefill is an adhesive, such as a resin, that serves to reinforce the physical and mechanical properties of the solder joints between the IC chip and the substrate. The underfill typically not only provides fatigue life enhancement of a packaged system, but also provides corrosion protection to the IC chip by sealing the electrical interconnections of the IC chip from moisture, oftentimes resulting in an improvement in fatigue life of ten to over one hundred fold, as compared to an un-encapsulated packaged system.
Heretofore, cycloaliphatic epoxies, typically combined with organic acid anhydrides as a hardener, have been used in flip-chip packaged systems as an underfill. This is primarily due to the low viscosity of cycloaliphatic epoxies prior to curing, as well as their acceptable adhesion properties after curing. Other epoxies such as bisphenol A or F type or naphthalene type can also be used in the underfill formulations. Additionally, silica has been utilized as a filler in these underfill formulations, i.e., up to 70% (by weight), in order to lower the CTE of the epoxy resin. By way of example, the material properties represented in Table 1 typically are exhibited by typical prior art epoxy underfill compositions.
Frequently, defects in a chip or circuit board are not discovered until after assembly of an electrical component using an underfill adhesive. Heretofore, it has generally been commercially impractical to separate and clean the chip and/or board so that the non-defective components can be reused. This results in increased production costs due to the waste of otherwise usable components. An effective way to address this problem is to make the flip-chip devices reworkable under certain conditions.
One method which has been developed in an attempt create a reworkable flip-chip device has been to incorporate a non-stick release coating on the boundary surface between a chip and a substrate. For example, U.S. Pat. No. 5,371,328 discloses a reworkable flip-chip type of circuit module using a non-stick release coating on all surfaces intermediate of the chip and the substrate. While this non-stick release coating may be suitable in some applications, it is likely that the use of such a release coating may reduce the adhesion of all the interfaces including those of the underfill to chip and underfill to substrate. These adhesions are important to the reliability of the flip-chip interconnections. Accordingly, this approach is not ideal for use in flip-chip applications.
Another approach to providing a reworkable flip-chip interconnection is to use a reworkable underfill. Presently, the materials that are undergoing development for reworkable underfills can be classified into two categories: chemically reworkable underfills and thermally reworkable underfills.
Chemically reworkable underfills generally require the use of harsh acids and/or bases. For example, U.S. Pat. No. 5,560,934, issued to Afzali-Ardakani et al., discloses epoxy compositions that are soluble in an organic acid after curing. Utilizing relatively strong chemicals such as acids (or bases) during reworking, however, oftentimes leads to a messy, time-consuming rework process. Additionally, it has been found that the use of chemicals during the rework process typically makes localized repair of a packaged system difficult and, sometimes, impossible. Therefore, it is believed that use of a thermal rework process would avoid these problems and offer the possibility of a quick, clean, and localized rework process.
U.S. Pat. No. 5,659,203, issued to Call et al., discloses a reworkable flip-chip module utilizing a specially defined thermoplastic resin as an encapsulant. The thermoplastic resin, such as polysulfone, polyetherimide, etc., possesses a high glass transition temperature (Tg), e.g., 120xc2x0 C. less than Tg less than 220xc2x0 C., and must be either dissolved in a solvent or heated above its melting point during the encapsulation process. Therefore, use of these thermoplastic resins as encapsulants for FCOB applications may be undesirable, since such applications typically require an underfill which is free of solvent and in liquid form during the encapsulation process, and typically require keeping the packaged system at lower temperatures in order to maintain the integrity of the eutectic solder which is utilized with the organic PWB.
U.S. Pat. Nos. 6,197,122 and 5,948,922, issued to Ober et al., disclose thermally reworkable underfill formulations based on thermally decomposable epoxies containting a secondary or tertiary oxycarbonyl (ester) moiety. However, a secondary or tertiary oxycarbonyl moiety typically can easily be cleaved by weak acid or base, and is sensitive to moisture. Also the epoxies containing a secondary or tertiary oxycarbonyl moiety typically have higher moisture uptake than a standard epoxy. All these facts indicate that epoxies containing a secondary or tertiary oxycarbonyl moiety might not be suitable for underfill application where high reliability is required.
Thus, it can be seen that none of the prior art methods are ideally suited for use as an underfill to bond chip and substrate to allow fast and efficient rework of FCOB devices without sacrificing the reliability of the devices. Therefore, it is desirable to provide an underfill composition that will not negatively affect the overall performance of the assembly, while still allowing cost effective and efficient rework.
Accordingly, it is an object of this invention to provide a polymeric composition that has properties suited to use as an underfill while also offering thermal reworkability. The present invention is focused on epoxy base materials because epoxy base materials have desirable properties for use as an underfill and are the only materials that have been proven to provide flip-chip devices with acceptable reliability.
Most epoxy materials are thermosetting compositions and are difficult or impossible to remove after curing. The present invention overcomes this limitation by developing new diepoxides that contain thermally degradable groups within their structures and using these new diepoxides in the epoxy formulations to make the thermoset network degradable at a desired temperature. This makes the new epoxy formulations reworkable. Moreover, these thermally degradable groups have good properties such as high moisture resistance, high chemical resistance and low moisture uptake so that they are suitable for underfill application. This improvement is advantageous in flip-chip application of epoxy compositions where epoxy materials are used as the underfill to reinforce the solder joints. Removal of the epoxy allows replacement of defective devices, saving the cost of discarding other valuable components in a microelectronic assembly.
There are two ways of developing reworkable epoxy base materials. One is to develop new epoxies that decompose at rework temperature. The other is to develop additives to add into the existing epoxy formulations that have previously been found suitable for use as underfill encapsulants. The present invention focuses on the first category and uses thermally degradable epoxies containing integral thermally cleavable groups that decompose at rework temperatures. The second category is the subject of U.S. Pat. No. 6,172,141.
The thermally cleavable groups of the present invention have been selected to meet the following criteria:
1. The cleavable groups should be sufficiently stable to permit the epoxy network to perform its function in a specific application;
2. The cleavable groups should be inert to the curing reaction of the epoxy network;
3. The cleavable groups should not adversely affect the overall properties of the epoxy network;
4. The cleavable groups should decompose quickly at elevated temperature so that they break down the structure of the epoxy network, leading to its easy rework.
5. The link should be stable in the environment to which the cured epoxy will be exposed.
6. The synthesis of the epoxides containing the cleavable link should be simple, with high yield, and cost effective.
The present invention discloses carbonate and carbamate epoxides which have been found to meet the above criteria. After introduction into the epoxy structure, the carbamate and carbonate groups do not significantly interfere with epoxy curing, nor do they adversely affect epoxy properties including Tg, modulus, CTE, adhesion. However, the existence of these groups inside the epoxy structure reduces the epoxy decomposition temperature from 350xc2x0 C. to as low as 200xc2x0 C. Optimal rework temperatures for flip-chip devices are generally between 200xc2x0 C. and 250xc2x0 C. because the eutectic solder reflow temperature is within this temperature region. Therefore, these two groups may be suitable for use in applications needing an epoxy which is reworkable around solder reflow temperature.
More particularly, the present invention is directed to a thermally reworkable epoxy composition for encapsulating and protecting an electronic device or assembly. The thermally reworkable epoxy composition includes the cured reaction product of: a cycloaliphatic epoxide containing either a carbonate or a carbamate group; an organic hardener; and a curing accelerator. The present invention is also direct to a method of protecting, encapsulating, reinforcing, assembling, or fabricating a device or a chemical product with a cured epoxy composition which is thermally reworkable, wherein the epoxy composition includes the reaction product of: a thermally degradable cycloaliphatic epoxide; an organic hardener; and a curing accelerator.